Question

The process by which a lake ecosystem is altered by eutrophication involves several stages, each causing the next. Which of the following stages would occur second in the chain of causation?
A. Algal blooms occur.
B. Oxygen levels drop in deeper water.
C. Phosphorus input from sewage and agricultural runoff increases.
D. Respiratory demand from decomposers increases.

Answer 1

Hint: Lake ecosystem comprises aquatic microorganisms and macroorganisms such as aquatic plants and animals. It is characterized by stable distribution and flow of nutrients. It is an example of lentic ecosystems.

Complete step by step solution:
Eutrophication is the process in which there is excess of nutrients in some parts or entire water body. It is characterized by high amounts of minerals, nitrogen and phosphorus.

It is a slow process in which there is gradual accumulation of nutrients on the water surface caused mainly due to overactive algae, fungi, phytoplanktons and other microorganisms.

The most common result of eutrophication is algal blooms. These can be harmful as they cover the surface of water and degrade the bacteria that could digest them. In addition, the light of the sun stops entering into the water so the phytoplanktons present in the water become unable to perform the function of photosynthesis. As a result, respiration also ceases or gets significantly reduced. This causes oxygen depletion in the water and leads to death of some fish and plants.

The process of eutrophication occurs in several stages. The different stages that occur are in the following sequence: C) Phosphorus input from sewage and agricultural runoff increases. A) Algal blooms occur. D) Respiratory demand from decomposers increases. B) Oxygen levels drop in deeper water.

Hence, the stage that would occur second in the process of causation would be, A., Algal blooms occur.

Note:
Water pollution also causes eutrophication. High amounts of sewage water, detergents, industrial wastes that are disposed of to water, chemical fertilizers which enter the water bodies due to run off etc going to water result in excess amounts of nitrogen and phosphates.